1. Field of the Invention
The present invention relates to a wide-band antenna; more particularly, the present invention relates to a wide-band planar antenna for wireless network communications.
2. Description of the Prior Art
As the physical Internet becomes more and more popular, people pay much attention to a wireless, long-distance, and wide-band network in place of the physical Internet to increase the popularity in wideband communications. Thus, more advanced wireless communication network technologies and standards continuously emerge. For example, Wi-Fi wireless network standard is previously defined in IEEE 802.11 by Institute of Electrical and Electronics Engineers (IEEE); Worldwide Interoperability for Microwave Access (WiMAX) is recently defined in IEEE 802.16. Especially for WiMAX, the transmission distance has been increased from meters to kilometers, and the bandwidth becomes wider over the prior art.
In order to comply with the progress of wireless communication network technology, the antenna needs to be enhanced for receiving/transmitting wireless signals accordingly. FIG. 1 shows a traditional dual-band antenna disclosed in the U.S. Pat. No. 6,861,986. The dual-band antenna includes a first radiator 31 and a second radiator 32, both connected to a ground 4. Signals are fed through a feed-in point 61 directly to excite the first radiator 31 to generate a high frequency band mode, whose central operating frequency is about 5.25 GHz. The direct fed-in signal can also excite the second radiator 32 to generate a low frequency band mode, whose central operating frequency is about 2.45 GHz. Furthermore, the length of the second radiator 32 is about one quarter (¼) of the wavelength at its operating frequency.
Because the antenna is fed with signals in a direct-feed-in manner, the bandwidth of the low frequency band mode is about 200 MHz, which cannot satisfy WiMAX requirement. Furthermore, in order to meet the operating frequency of the low frequency band mode, the length of the second radiator 32 cannot be further reduced resulting in the restriction of miniaturization of the electronic devices.